2-ethylhexanol is produced industrially on a large scale. To manufacture the 2-ethylhexanol, two molecules of n-butyraldehyde undergo aldol condensation and dehydration to give 2-ethylhexenal which is also known as ethylpropylacrolein. This is then subjected to hydrogenation to provide crude 2-ethylhexanol. Generally the hydrogenation reaction does not go to completion and the crude product will not only contain unreacted 2-ethylhexenal but may also contain one or both of 2-ethylhexenol and 2-ethylhexanal i.e. the compounds in which only one of the unsaturated group and the carboxyl group has been hydrogenated.
Since the desired 2-ethylhexanol is mainly used to make esters such as dioctyl phthalate which serve as plasticizers in the production of polyvinyl chloride, it is necessary to produce the 2-ethylhexanol with a high level of purity.
One measure of the purity is the colour of the product or rather the absence of colour in the product. An indication of the desired level of purity is that the product not only has low intrinsic colour, it must also produce a low colour in a standard test after being boiled with sulphuric acid. One such test is set out in BS 4583. In this test the sample is treated with concentrated sulphuric acid under conditions specified in the Standard. Any impurities in the sample form coloured compounds by reactions which are catalysed by the sulphuric acid. Once the sample has been reacted with the sulphuric acid, the colour is compared with that of a series of platinum cobalt standards of equal volume in matched 100 ml Nessler tubes. These standards are also known as APHA or Hazen standards. The APHA standard relates to the US standard ASTM D1209. By the use of this test an accelerated determination is provided of any undesirable colour which may be produced when the 2-ethylhexanol is reacted with acids such as phthalic acid to produce esters.
It is the presence of unreacted 2-ethylhexenal, and any partially hydrogenated 2-ethylhexenol and/or 2-ethylhexanal which cause the undesirable results and the sulphuric acid test will identify their presence.
Similar issues are noted with the production of other 2-alkylalkanols such as the production of 2-propylheptanol from n-valeraldehyde where the product comprises unreacted valeraldhyde and partial hydrogenation products. The n-valeraldehyde starting material will generally comprise up to about 10% of other C5 aldehydes. Where these are present, the product will comprise these other C5 aldehydes.
A conventional process for the production of 2-alkylalkanols such as 2-ethylhexanol or 2-propylheptanol is illustrated schematically in FIG. 1. In this process the starting aldehyde is passed in line 1 to reactor 2 where an aldol condensation reaction is carried out to form an unsaturated aldehyde, i.e. an acrolein. The by-product water, which is generally separated by decantation, is removed in line 3. The acrolein is then passed in line 4 to a hydrogenation reactor 5 to which hydrogen is added in line 6. The hydrogenation may be carried out in the liquid or vapour phase. A catalyst, such as one containing one or more Group VI-X metals such as copper, chromium, nickel, zinc, iridium or ruthenium, will generally be used. Suitable hydrogenation catalysts will generally include those available from Johnson Matthey as the Pricat and HTC range.
The product stream recovered from the hydrogenation reactor 5 comprising some alkylalkanol is recovered in line 7. This stream will generally also comprise small quantities of unsaturated alcohol and saturated aldehyde. Some unreacted unsaturated aldehyde may also be present. The stream is therefore passed to a polishing hydrogenation zone 8 where it is contacted with hydrogen supplied in line 9 to further hydrogenate the unsaturated alcohol and saturated aldehyde. Catalyst will generally also be used for the polishing hydrogenation.
A product stream is recovered from the polishing hydrogenation reactor 8 in line 10 and is then subjected to a two stage distillation process. In a first distillation zone 11, lights are removed in line 12. A stream having a reduced lights concentration is then passed in line 13 to a second distillation zone 14 where the heavies are separated and removed in line 15. The product stream of alkylalkanol is recovered in line 16.
Various modifications of the hydrogenation and purification sequences have been suggested with a view to improving the quality of the product, improving catalyst life or both improving the quality of the product and improving catalyst life.
DE 1003702 describes a process in which a low pressure hydrogenation reaction is carried out. This leaves from 4 and 15% aldehyde unconverted. The product of this first hydrogenation reaction may optionally be subjected to distillation to remove light compounds such as 2-ethylhexanal. The product of the first hydrogenation from which at least some light compounds have been removed is then subjected to a high pressure hydrogenation to remove the lights and heavies. Whilst the acid colour of the product is not specifically described, it is described that the carbonyl number is 0.1 and it can therefore be inferred that the acid colour will be above 60.
An alternative arrangement is described in GB1252678. In this arrangement, the product of the aldolisation is subjected to distillation to remove heavies before it is passed to the hydrogenation reaction.
A further process is illustrated in U.S. Pat. No. 7,663,006. Here there is a suggestion that dihydropyrans are acid colour making by-products. It is further commented that it is difficult to separate these dihydropyrans from 2-ethylhexenal by distillation and it is therefore proposed that their presence in the purification step should be limited. In one arrangement it is suggested that this is achieved by distillation of the feed to the hydrogenation reactor to remove heavies or a two stage hydrogenation reactor using different catalysts. The process described in U.S. Pat. No. 7,663,006 is illustrated schematically in FIG. 2.
In this process the starting aldehyde is passed in line 21 to reactor 22 where an aldol condensation reaction is carried out to form an unsaturated aldehyde, i.e. an acrolein. The by-product water is separated by decantation and removed in line 23. The acrolein is then passed in line 24 to a first distillation zone 25 where heavies are separated and removed in line 26. The stream of the unsaturated aldehyde having a reduced heavies content is passed in line 27 to hydrogenation reactor 28 to which hydrogen is added in line 29. The product stream recovered from the hydrogenation reactor 29 is recovered in line 30. This stream will comprise some alkylalkanol and will generally also comprise small quantities of unsaturated alcohol and saturated aldehyde. Some unreacted unsaturated aldehyde may also be present. This stream is then passed to a polishing hydrogenation reactor 31 where it is contacted with hydrogen supplied in line 32 to further hydrogenate any unreacted unsaturated aldehyde and the unsaturated alcohol and saturated aldehyde.
A product stream is recovered from the polishing hydrogenation reactor 31 in line 33 and is then subjected to a two stage distillation process. In a second distillation zone 34, lights are separated and removed in line 35. A stream having a reduced lights concentration is then passed in line 36 to a third distillation zone 37 where the heavies are separated and removed in line 38. The product stream is recovered in line 39.
Whilst this process may assist in the removal of colour forming compounds, the requirement for a distillation zone between the aldolisation reactor and the hydrogenation reactor together with the requirement for two reaction zones after the polishing hydrogenation reactor adds to the capital and operating costs of the process.
In addition, carrying out distillation on the stream comprising the unsaturated aldehyde, which is generally a vacuum distillation, is disadvantageous since air leakage can occur which can cause the formation of acids and aldehydes which are known to produce heavy compounds from condensation reactions when heated.
It is therefore desirable to provide a process which enables 2-alkylalkanols to be produced with a low acid colour and with a minimum number of processing steps to reduce capital and operating costs. It is also desirable that at least some of the other problems noted with prior art processes are at least partially overcome and preferably obviated.